That most exotic diseases occur outside the US relegates infectious disease physicians to treating scores of patients with MRSA; diabetic foot infections; pneumonia; urinary tract infections; and the occasional zebra.

One domestic disease that packs all of the punch of its more well known foreign brothers-in-arms, however, is the rodent-associated hantavirus.

Hantavirus Pulmonary Syndrome (HPS) first came to wide notice after a headline-grabbing outbreak occurred in the Navajo-inhabited Four Corners region of the western US in 1993 (see the book Of Mice, Men, and Microbesfor a great overview). Since that time, sporadic cases have occurred including an outbreak last year at Yosemite National Park.

A recent paper authored by CDC officials details 20 years of hantavirus infections in the US. Some important points include that a total of 624 cases occurred since 1993 and 96% of cases occurred west of the Mississippi River.

The aspect of hantavirus that I find most fascinating is the association of outbreaks with high amounts of rainfall. When high levels of rainfall occur, such as during an El Niño, rodent populations surge because of the abundance of food available. Such rodent population surges make contact with humans much more common and, consequently, human cases of hantavirus occur. In fact, this correlation was known in Navajo medical traditions long before recent outbreaks.

Though rare, it is important to consider HPS in patients with unexplained pulmonary edema and fever, especially if hemoconcentration is present and the patient is relatively young.